Automatic sensitivity tuning system



Jan. 17, 1939. A. w. BARBER AUTOMATIC SENSITIVITY TUNING SYSTEM Filed June '7, 1935 Fig.4

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UNITED STATES PATENT OFFICE 6 Claims.

My present invention relates to a novel system for tuning automatic volume control radio receivers and, more particularly, to a method of, and means for, providing a system by which such receivers may be accurately tuned by ear.

One object of my invention is to provide, in

automatic volume control radio receivers, a system which makes it possible to tune such receivers by ear. Another object is to render automatic volume control receivers apparently selective, especially to strong signals during tuning. Still another object is to restore this tuning system to its initial condition after tuning to a station, rendering it ready for immediately tuning to any other station. I In the days of low sensitivity, low selectivity radio receivers, the tuning of a receiver was a relatively simple matter. Actually even a mistuned receiver operated quite well, the only detrimental result being a reduction in volume of output. With the advent of high degrees of selectivity in receivers, accurate tuning became necessary as mistuning resulted in poor tone quality. Still it was easy to tune by ear by tuning for maximum output signal. However, with the addition of auto-matic volume control, the tuning problem was greatly increased, as tuning by ear to maximum output was no longer satisfactory due to the action of the control in counteracting changes in signal when passing thru resonance. The output of such a receiver remains substantially constant over a wide range on either side of true resonance but the quality of reproduction is adversely affected when the tuning is off resonance. In order to make tuning possible in such receivers, various devices have been resorted to such as tuning meters, neon lamps and the like, which operate to give a visual indication of tuning. While visual tuning is possible, it departs from the more familiar and natural method of tuning by ear. My, present invention deals with a method of tuning an automatic volume control receiver by ear. My method in addition requires fewer and less expensive accessories than visual tuning systems now in use.

Briefly my invention consists in increasing the time constant of the automatic volume control system during at least the final process of tuning. The effect is that the automatic volume control system ceases to follow the signal variations due to tuning and hence permits tuning by car, since under these conditions the receiver regains its true selectivity.

My method of efiectively increasing the time 55 constant of the automatic volume control circuit is to add a large condenser to the system connected between some point carrying the automatic volume control voltage and ground. A pushbutton makes a convenient means of connecting this condenser although other suitable switch means may be used. In order to make the system self-restoring, I provide a discharge circuit for the condenser which operates automatically when the delay circuit switch is released. Without this restoring action strong signals would build up a charge in the delay circuit which would remain and prevent tuning to weaker'stations.

The novel features characteristic of my invention are set forth in particular in the appended claims. The organization and method of operation of the invention itself will best be understood by reference to the following description taken in connection with the drawing in which I have indicated by means of circuit diagrams and explanatory curves how my invention may best be carried into effect.

In the drawing,

Fig. 1 shows a detector circuit embodying a simple form of my invention.

Fig. 2 shows a restoring circuit added to the simple form of my invention.

Fig. 3 shows a modified circuit whereby audio frequency currents may be generated.

Fig. 4 shows several selectivity curves useful in explaining the operation of my invention.

My tuning system is intended for use primarily in the case of radio receivers of the automatic volume control type. In Figs. 1, 2 and 3 I have shown only simplified detector circuits as it will be evident to one skilled in the art how these detector circuits may be embodied in radio receivers. It will also be evident that while simple diodes are shown that they may represent merely the automatic volume control voltage generating portions of complex diode or other detector circuits as are well known in the art.

In Fig. 11 have shown my invention embodied in a simple diode rectifier circuit. A thermionic vacuum tube diode I having electron emitting cathode 2, cathode heater 3 and anode 4 receives the unrectified output of a carrier amplifier, not shown, by means of input coil 5 which may be tuned, as shown, by condenser 6. An output or rectified current load circuit consisting of resistor I is provided in series with the diode and input coil. A condenser 8 is shunted across load resistor I to provide a low impedance path for currents of carrier frequency between diode and input coil. Cathode 2 is shown grounded at 9 which is intended as a reference point or a point 5 of zero potential. Under these conditions, when an input voltage is received from the carrier amplifier thru coil 5, a direct current will flow in resistor l in such a direction as to make the coil end negative with respect to the tube end as indicated. The magnitude of this current and hence the voltage across resistor I, will be proportional to the amplitude of the input carrier voltage and hence may be used for automatic volume control in ways well known in the art, by returning the voltage from the coil end of resistor 1 to control points of amplifier which are, in general, one or more control grids of carrier voltage amplifying tubes. The system so far described forms the usual automatic volume control circuit. The time constant of condenser 8 and resistor I will usually be a fraction of a second and of the order of from one tenth to one half second, as the action of the control is largely determined by the rectifier circuit and should operate to counteract fading but should not attenuate low frequency modulations.

At this point reference is made to Fig. 4 in order to explain the operation of Fig. 1. Curve ab shows the selectivity of a modern radio receiver to which my automatic volume control system of Fig. 1 may be applied. However, if this receiver is tuned by ear, the apparent selectivity will be this true selectivity only if the signal is below the point where the automatic volume control starts to operate on the receiver gain. For a signal of moderate intensity the apparent selectivity of the receiver when tuned by car will be that shown by curve cd of Fig. l. II o is the point of. true resonance, as the receiver is tuned in either direction from o the signal decreases due to the actual circuit selectivity ab but the automatic volume control counteracts any change in output producing the apparently broadened selectivity cd. For still stronger signals more apparent broadening is produced as in curve cf. It wih be apparent thatwhen tuning under these conditions by ear that the true resonance point 0 will be very difiicult to locate.

Returning to a consideration of Fig. l I have found that a condenser it having a capacity large compared to condenser 85 may be connected across resistor 'i by means of push-button ii in order to fix the receiver sensitivity while making the final tuning. adjustment in tuning to a station. Condenser l6 charges quickly thru the low diode resistance, which is usually of the order ofone thousand ohms, butdischarges very slowlythru high resistance Tl usually one hundred thousand ohms or more. Thus the receiver sensitivity becomes effectively fixed and does not follow the signal variations caused by tuning over the receiver resonance curve. Under this fixedsensitivity condition the receiver becomes apparently as selective as its true selectivity as shown by curve ab of Fig. 4. It is now possible to tuneby ear as even slight mistuning greatly afiects the output volume of sound. When the tunin process has been completed, the pushbutton may be released and the automatic volume control system will operate in a normal manner following fading variations and the like. Other suitable switching means may be substituted for push-button i i within the spirit of the invention.

While in no way intended to limit the scope of this invention I have found the following constants satisfactory in the operation of my invention: resistor lequal to i megohms, condenser 23 equal to 250 micro-microfarads, condenser l0 equal to 4 microfarads.

In the operation of the system shown in Fig. 1 one disadvantage is noted, namely, that after tuning to a station if it is desired to tune to a weaker station the charge on condenser it] may not have had time to leak ofi and when the push-button is operated, the receiver is rendered too insensitive to tune to the weaker station. To overcome this efiect I have used a circuit as shown in Fig. 2. This circuit is similar in con,- nection and operation to Fig. l but with the discharge resistor l3 provided which is automatically connected across condenser it) when the double throw push-button i2 is released. Resister is may be of the order of one thousand ohms and its function is to discharge condenser IE quickly upon the release of push-button l2 thereby returning the fixation circuit to initial conditions rendering it ready to operate in tuning to any signal regardless of its intensity.

While I prefer the circuits of. Figs. 1 and 2 they are not adapted to generate audio frequency currents in the same rectifier path with the automatic volume control voltage. do this, the circuit of Fig. 3 may be used. Fig. 3 is similar to Fig. 2 except that resistor I l is interposed between resistor l and the point at which condenser H) is connected to the automatic volume control circuit. The result is the separation of the load circuit, consisting of resistor l and condenser 8, and the automatic volume control in such a way that the audio currents may be used as indicated to operate an audio amplifier by the rectified currents across resistor 1'. In Fig. 3 resistor 7 would in general be much smaller than the 4 megohms recommended above and nearer to one hundred thousand ohms. Resistor M may be several hundred thousand ohms or more as its function is that of. a filter between the audio and automatic volume control circuits.

t will be apparent to one skilled in the art that while I have described only a few systems whereby my invention may be carried into efiect, that it is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a carrier wave receiver, automatic volume control voltage generating means comprising a diode rectifier, an. input circuit to said diode, an output circuit associated with said diode for developing said automatic volume control voltage, said output circuit comprising a resistanceand condenser connected in parallel, means for delaying the decay of automatic volume control voltage developed in said output circuit for at least one second comprising a second condenser large compared to said first condenser and manually operable means for connecting said second condenser in parallel with said first condenser.

2. In a carrier wave receiver, the combination of a carrier current amplifier and an automatic volume control means, said automatic volume control means comprising a rectifier and a filter, means for connecting a condenser in parallel with said filter and means for discharging said condenser when not connected to said filter.

3. In a carrier wave receiver, automatic volume control means comprising a rectifier con- If it is desired to nected in series with an input circuit and an output circuit load resistor, additional means comprising an automatic volume control filter circuit, means for increasing the time constant of said filter circuit comprising a condenser, means for discharging said condenser comprising a second resistor and means for connecting said condenser either to said filter or to said discharge resistor.

4. In a carrier wave receiver, an automatic volume control system, said system comprising control voltage developing means and additional selectable means for maintaining said control voltage at essentially the maximum value developed by the action of any desired carrier wave signal on said volume control means for at least one second said selectable means comprising an electrical condenser and in addition a circuit associated with said selectable means for quickly dissipating any voltages existing in said selectable means.

5. In combination, a carrier current amplifier, a rectifier including input and. output circuits, means for applying the output of said carrier current amplifier to said rectifier input circuit, means for determining the normal time constant of said rectifier output circuit including at least one electrical condenser, means for altering said normal time constant to a second time constant of at least several seconds, said time constant altering means comprising at least an electrical condenser and a switch in series connection, a resistor, and means for connecting said second condenser across said last resistor when said second condenser is disconnected from said control circuit.

6. An automatic volume controlled carrier wave receiver comprising, a carrier current amplifier, a rectifier generating currents proportional to the average of the positive values of the currents traversing said amplifier, a circuit connected between said rectifier and at least one point in said amplifier whereby the gain of said amplifier is decreased in accordance with the currents generated by said rectifier, means associated with said rectifier comprising a double position switch connecting a condenser in parallel with the gain control generating portion of said rectifier when in one position and a resistor in parallel with said condenser when in the other position.

ALFRED W. BARBER. 

